Electroplating titanium ano titanium alloys



temperatures above 1000 United States Patent O ELECTROPLATING TITANIUM ANO TITANIUM ALLOYS David Halpert, Philadelphia, Pa., assignor to Vertol Aircraft Corporation, Morton, Pa., a corporation of Pennsylvania No Drawing. Application October 26, 1956 Serial No. 618,656

2 Claims. (Cl. 204-32) This invention relates to the plating of metals and more particularly to an aqueous electrolyte and process for electrodepositing an adherent nickel and/or cobalt coating on titanium and its alloys.

Titanium exhibits severe galling tendencies and lacks wear resistance thus presenting many problems in its fabrication. It is also an unusual characteristic of titanium that the most commonly used lubricants do not adhere to its surface and quite moderate loads are sufficient to cause high friction and galling between the parts. Further, when titanium and some of its alloys are heated in air to F., they are subject to embrittlement by oxygen and nitrogen.

It is therefore a primary object of this invention to apply an adherent protective coating on titanium and its alloys that will provide a hard bearing surface which is abrasion resistant and easily lubricated and which will protect the titanium from contamination when subjected to temperatures above 1000 F.

It is a further object to electrodeposit an adherent nickel and/or cobalt coating on titanium and its alloys that may be utilized as an undercoat upon which subsequent coatings can be applied by any of the standard plating processes.

A still further object of this invention is to provide a plating bath comprising an aqueous solution of a nickel and/or cobalt salt and a soluble inorganic acid wherein the composition agents are readily controllable at room operating temperature of the solution.

Further objects and advantages of the invention will become apparent from the following description.

It was found that conventional methods of plating were not successful when used with titanium and that the present known electrolytic plating solutions for depositing nickel coatings on metals did not form a completely adherent protective coating on titanium that would remain intact when subjected to flexing, bending and sustained heating at temperatures above 1000 F. This failure in forming the desired adherent coating may have been due to the presence of fluoride or chloride ions in the electrolyte since is was discovered that a firmly adherent coating could be applied to titanium and its alloys when the fluoride and chloride ions were generally absent from the electroplating solution.

It was further discovered that an adherent coating of nickel could be electrodeposited on titanium and its alloys from an aqueous electrolyte containing the sulfate salt of nickel in an amount approximately equivalent to that obtained with 70 to 375 grams per liter of nickel sulfate (NiSO -6H O) with the addition of sulfuric acid in quantity suflicient to adjust the solution pH value from about to 2. Also, cobalt or a nickel-cobalt alloy can be electrodeposited from an electrolyte solution essentially similar to that set forth for nickel except that the sulfate salt of cobalt (CoSO -7H O) is substituted for the nickel sulfate or added thereto in proportion to the percentage of deposited cobalt desired. The preferred proportion of the constituents per liter of plating solution consists of 300 grams of nickel and/ or cobalt sulfate dissolved in distilled or deionized water with the addition of sulfuric acid in a quantity sutlicient to adjust the solution pH value to about 1.7. During plating operations the electrolyte temperature is preferably maintained at room temperature and a plating current is passed through the member to provide thereon a current density of from 5 to 40 amperes per square foot, hereinafter referred to as A.S.F.

Increasing the temperature of the electrolyte up to as much as 160 F., with the composition thereof remaining the same or being varied by means of lesser quantities of sulfuric acid to adjust the pH value to as much as 6.2 accelerates the rate of covering and permits the use of higher current densities, in the neighborhood of A.S.F., Without burning the deposit.

For preparing the surface of titanium and its alloys prior to electroplating operations, the member should be thoroughly cleaned and may be so cleaned by a standard vapor degreasing solvent such as trichlorethylene or perchlorethylene followed by immersion in an alkaline solution containing 8 to 12 oz./gal. of caustic soda and 4 oz./ gal. of trisodium phosphate or other standard alkaline cleaner. This should be followed by immersion in a deoxidizing solution comprising 15 milliliters of hydrofluoric acid (48% concentrate) per liter of water until the evolution of gas from the surface of the titanium takes place and a violet or purple film believed to be titanium fluoride forms on the surface. The titanium member is then removed from the deoxidizing bath, thoroughly rinsed in water and then made the cathode in the electrolyte.

Preferably, a soluble anode member of commercially pure nickel is used in order that the metal ion concentration in the plating solution is replenished as the metal ions in solution are converted into a solid crystalline deposit on the titanium surface. However, if an insoluble anode is employed, the soluble sulfate salt of the deposited metal may be introduced into the plating solution for replenishing the supply of ions as they are plated out so as to maintain the desired metal ion concentration of the solution.

A plating current density of approximately 20 A.S.F. applied to a titanium member immersed in the electrolyte at room temperature results in a visually noticeable deposited coating within approximately 3 minutes. Although longer plating times may be desired for depositing proportionately thicker coatings, it was found that a 5 minute plating time produced an adherent coating of thickness sufl'icient to function as an undercoat for subsequent coatings of other metals applied by standard plating methods. Similarly, a plating current density of approximately 35 A.S.F. applied to a titanium alloy member immersed in the electrolyte at a temperature of F. and comprising 300 grams of cobalt sulfate per liter of solution and sulfuric acid in quantity sufiicient to adjust the pH value thereof to 6, resulted in a visually noticeable coating within approximately 30 seconds. A longer plating time of approximately 2 minutes resulted in an adherent cobalt coating of thickness sufficient to function as an undercoat for a subsequent coating of chromium applied by a standard plating method.

Having thus described by invention, what I claim is:

l. The process of electrodepositing nickel on titanium and titanium base alloy members which consists of immersing the titanium member in a solution of hydrofluoric acid for a time suflicient to form a violet film, rinsing said titanium member in water, and then immersing the member in an aqueous electrolyte consisting of from 70 to 375 grams of nickel sulfate (NiSO -6H O) per liter of solution and sulfuric acid in quantity suflicient to Patented Jan. 19, 19.60.

adjust the solution pl-I value from about to said bath remains firmly adherent at a temperature of 1000 2. The process of electrodepositing metal selected from the group consisting of nickel, cobalt and alloys of nickel and cobalt on titanium and'titanium base alloy members which consists of immersing the titanium member in a solution of hydrofluoric acid for a time sufficient to form aviolet film, rinsing said titanium member in water and then immersing the member in an aqueous electrolyte consisting of at least one sulfate salt selected from the group consisting of nickel sulfate and cobalt sulfate, the nickel sulfate in an amount corresponding, to from 70 to 375-grams of nickel sulfate (NiSO -6H 0) per liter of solution, the cobalt sulfate in an amount corresponding to" from 70 to 375 grams of cobalt sulfate (CoSO -7-l-I 'O) per liter of solution, and sulfuric acid in quantity suflicient to adjust the solution pH value from about 0 to 6.2,

said bath being substantially free of metal salts of halogen acids, and then passing a plating current through the member at a current density of 5 to A.S.F., thereby depositing on the titanium member a metal coating which remains firmly adherent at a temperature of 1000 F.

References Cited in the file of this patent UNITED STATES PATENTS 665,915 Kugel Jan. 15, 1901 1,003,092 a Dow etal Sept. 12, 1911' 1,371,414 Edison Mar. 15, 1921 2,240,805 Semon May 6, 1941 2,619,454 Zipponi Nov. 25, 1952 2,646,396 Dean July 21, 1953 2,654,703 Brown .v Oct. 6, 1953 2,711,364 Beach June 21, 1955 2,711,389 Beach et a1 June 21, 1955 OTHER REFERENCES Principles of Electroplating and Electroforrning, Second- Edition; Blum et a1. (1930). McGraw-Hill Book Company Inc., N.Y., pages 240-248. 

2. THE PROCESS OF ELECTRODEPOSITING METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL, COBALT AND ALLOYS OF NICKEL AND COBALT ON TITANIUM AND TITANIUM BASE ALLOY MEMBERS WHICH CONSISTS OF IMMERSING THE TITANIUM MEMBER IN A SOLUTION OF HYDROFLUORIC ACID FOR A TIME SUFFICIENT TO FORM A VIOLET FILM, RINSING SAID TITANIUM MEMBER IN WATER AND THEN IMMERSING THE MEMBER IN AN AQUEOUS ELECTROLYTE CONSISTING OF AT LEAST ONE SULFATE SALT SELECTED FROM THE GROUP CONSISTING OF NICKEL SULFATE AND COBALT SULFATE, THE NICKEL SULFATE IN AN AMOUNT CORRESPONDING TO FROM 70 TO 375 GRAMS OF NICKEL SULFATE (NISO4.6H2O) PER LITER OF SOLUTION, THE COBALT SULFATE IN AN AMOUNT CORRESPONDING TO FROM 70 TO 375 GRAMS OF COBALT SULFATE (C3SO4.7H2O) PER LITER OF SOLUTION, AND SULFURIC ACID IN QUANTITY SUFFICIENT TO ADJUST THE SOLUTION PH VALUE FROM ABOUT 0 TO 6.2, SAID BATH BEING SUBSTANTIALLY FREE OF METAL SALTS OF HALOGEN ACIDS, AND THEN PASSING A PLATING CURRENT THROUGH THE MEMBER AT A CURRENT DENSITY OF 5 TO 100 A.S.F., THEREBY DEPOSITING ON THE TITANIUM MEMBER A METAL COATING WHICH REMAINS FIRMLY ADHERENT AT A TEMPERATURE OF 1000*F. 